Current sensing diagnostic circuits are often used in appliances (e.g., refrigerators) to monitor the operation of electrical components such as, for example, relay circuits. Such a current sensing diagnostic circuit is illustrated in simplified schematic form in FIG. 1. If a current begins to flow through the diagnostic circuit when the relay is commanded to close, a sensor detects that current and reports that the electrical component being monitored is operating as intended. If, however, current does not flow through the diagnostic circuit when the relay is commanded to close, the sensor notes the absence of that current and reports to the control and/or microprocessor that the electrical component is not functioning properly. When the electrical component is not working, malfunctioning, and the like, a technician is often summoned to repair and/or replace the electrical component in the appliance. Unfortunately, indication of a failure of the electrical component to function properly can occur when a variety of different faults (e.g., an open load, a disconnected wire, and the like) are experienced and/or the electrical component itself is damaged. Therefore, the technician will have to check a number of different potential problems to determine which electrical component has actually failed, which electrical component needs to be replaced, which leads or connections to check, and the like.
Another method is to look for line voltage at the output of the relay. Referring to FIGS. 2–5, one such conventional voltage sensing diagnostic circuit 10, as known in the art, is illustrated. The known diagnostic circuit 10 includes a variety of resistors 12, 14, 16, 18, and a capacitor 20 coupled together as shown to sense the applied voltage from the relay circuit 22 to the load 24. The diagnostic circuit 10 is typically coupled to a sensing circuit 30 at node 32, which provides the diagnostic information to a controller or microprocessor 42.
The relay circuit 22 includes an electromagnetic coil 34 and a switch 36. The electromagnetic coil 34 is coupled to a direct current power source 38 and a driver circuit 40. The driver circuit 40 is typically commanded by the microprocessor 42 to switch the alternating-current (AC) power source 44 to the load 24.
Referring specifically to FIG. 3, when the electromagnetic coil 34 is de-energized and the switch 36 is open, the diagnostic circuit 10 does not sense any voltage applied to the load 24. This is the normal sense for a relay open condition using this type of current sense circuit 10. Unfortunately, since this conventional circuit 10 detects voltage applied to the load 24, and since the opening of the relay switch 36 ensures that no voltage is applied to the load 24, the circuit 10 does not detect anything different at this point if there is a fault (open circuit) in the wiring to the load as illustrated in FIG. 3 or if the load itself is disconnected. The same can be said for the current sense circuits.
When the relay 22 is closed under normal operation as shown in FIG. 4, the sensing circuit 30 detects the current flow to the load 24. The sensing circuit 30 then communicates that the relay circuit 22 is operating normally as commanded. However, because the circuit 10 is configured to monitor the current flow to the load 24 to determine if the relay 22 is operating properly, a broken wire to the load 24, as illustrated in FIG. 5 (or a disconnection of the load 24 itself) will be flagged as a failure of the relay 22. Unfortunately, this may well lead the technician to replace the relay or the relay board, when in fact the relay 22 and its driver 40 are operating properly. Further troubleshooting would then be required to isolate the problem detected by circuit 10.
For the convenience of the reader, a summary of the operation of the known diagnostic circuit 10 is provided in FIG. 6. As illustrated in the first two rows of the truth table, anytime the switch 36 in relay circuit 22 is open, whether or not the load 24 is connected or unconnected, the sensing circuit 30 sees no current. In contrast, as shown in the third row of the truth table, when the switch 36 in relay circuit 22 is closed and the load 24 is connected, the sensing circuit 30 sees the current flow to that load. This is interpreted as normal relay operation. However, even if the relay 22 operates normally and closes its switch 36, if the load is disconnected, the circuit 10 does not see current flowing to the load. This is reported as a failed relay 22, even though it operated to close its switch 36 when commanded. As a result, faults in the relay circuit 22, the load 24, or elsewhere are difficult to locate, pinpoint, and troubleshoot.
Therefore, a diagnostic circuit that can indicate whether a load is present or missing, as well as determine if a relay circuit is functioning properly, would be desirable. The invention provides such a diagnostic circuit. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.